In optical communications, optical signals carry information. For example, a transmitter (e.g., a laser or laser diode) in an optical or optoelectronic transceiver converts one or more electrical signals into optical signals, and a receiver (e.g., a photodiode) in an optical or optoelectronic transceiver converts one or more optical signals into electrical signals. One objective of optical communication research and development is to increase and/or maximize bandwidth (e.g., the amount of information transmitted) to the greatest extent possible.
At a given baud rate, the capacity of a transmitter or receiver in an optical transceiver is limited by the number of optical channels (or wavelengths for wavelength division multiplexing [WDM] systems) that one transmitter optical subassembly (TOSA) or receiver optical subassembly (ROSA) can contain. When conventional optical components are used to multiplex (mux) and/or demultiplex (demux) a multi-channel optical signal, a minimal size (e.g., compactness) and reliability are primary concerns about the system processing such signals.
FIG. 1A shows a multiplexer 10 for a conventional 4-channel transmitter, comprising first through fourth filters 32, 34, 36 and 38 at one end and a mirror 40 at an opposite end. The first filter 32 may be a high pass, low pass or bandpass filter. The second through fourth filters 34, 36 and 38 are wavelength-selective filters or beam combiners. Thus, in one embodiment, each of the first through fourth filters 32, 34, 36 and 38 is a bandpass filter. The first through fourth filters 32, 34, 36 and 38 and the mirror 40 may be oriented at an identical angle with respect to the optical signals 12, 14, 16 and 18 passing through respective first through fourth lenses 22, 24, 26 and 28.
The multiplexer 10 combines the first through fourth optical signals 12, 14, 16 and 18 to form a multi-channel signal 19. After passing through the first filter 32, the first optical signal 12 is reflected by a mirror 40 to a location or spot on the second filter 34 where it is combined with the second optical signal 14 to form a first combined signal 15. The first combined signal 15 is reflected by the mirror 40 to a location or spot on the third filter 36 where it is combined with the third optical signal 16 to form a second combined signal 17. The second combined signal 17 is reflected by the mirror 40 to a location or spot on the fourth filter 38 where it is combined with the fourth optical signal 18 to form the multi-channel signal 19, which is output through an output port to a transmission medium (e.g., an optical fiber).
The zig-zag shape of the optical paths of the optical signals 12, 15, 17 and 19 combined with the filter array 32-38 is the simplest way to mux optical signals from lasers having different wavelengths into a single fiber, as shown in FIG. 1A. More bandwidth can be added to the transmitter by increasing the number of optical channels in the multiplexer. For example, the multiplexer 50 of FIG. 1B includes 8 channels 51-58 at 8 different wavelengths, respectively passing through eight lenses 61-68 and eight filters 71-78, the first seven of which are reflected by a mirror 80 to form an 8-channel optical signal 59.
However, when the number of wavelengths increases, the size of the system grows considerably, both in the transverse and longitudinal directions. If a single block 50 is used to mux all 8 channels 51-58, it may cause some issues. For example, the optical path difference becomes relatively large between the first channel 51 and the last channel 58, making the design and alignment of the lenses 61-68 more difficult. Any pitch error accumulates over the increased number of channels 51-58. For example, if the first channel 51 has a small error, it will be 8 times greater at the last channel 58. In other words, it becomes more difficult to ensure good performance for each channel.
Furthermore, since the structural block for the multiplexer 50 has an alignment axis in the plan view (FIG. 1B) that is not at a right angle, the length of the structural block also increases with increasing channel number, which is highly undesirable given the fixed dimensions of standard optical transceiver packages. The system is also more vulnerable to index variations due to changes of temperature and/or pressure.
There are a few methods to reduce the overall dimensions of the system, but each method has its drawbacks. For example, one may remove the structural block 50 that holds the filters 71-78 together (e.g., using air between the filters). This reduces both the length and the width of the system. However, without a solid piece to provide reference surfaces for the other optical components, it is generally very difficult to place the filters 71-78 in precise positions, as designed. Also, the stability of the system is adversely affected, since the contact area between the filters 71-78 and the optical bench (e.g., the substrate on which the optical components are placed) is much smaller than the contact area between the structural block 50 and the optical bench, as well as the contact area between the filters 71-78 and the structural block 50.
Another method to reduce the dimensions of the system is to use a low refractive index material for the structural block 50. The lowest refractive index of an available transparent material is about 1.3, which means the dimensions are not reduced by much (e.g., relative to glass or quartz). Meanwhile, using a special refractive index material increases the cost of the multiplexer.
Yet another method to reduce the dimensions of the system is to increase the angle of the structural block 50. This reduces the length of system, but does not reduce the width by much, since the angle between the outermost signal 51 and the corresponding wall of the structural block 50 also increases. Besides, coatings at a large angle relative to an incident beam are more sensitive to the angle of the incident beam, therefore causing potential tolerance issues.
This “Discussion of the Background” section is provided for background information only. The statements in this “Discussion of the Background” are not an admission that the subject matter disclosed in this “Discussion of the Background” section constitutes prior art to the present disclosure, and no part of this “Discussion of the Background” section may be used as an admission that any part of this application, including this “Discussion of the Background” section, constitutes prior art to the present disclosure.